Polymer electrolytes comprising polymers having ion-conductivity are used as diaphragms of electrochemical devices such as primary batteries, secondary batteries and solid polymer type fuel cells. For example, perfluorosulfonic acid-based materials such as Nafion (trademark of DuPont de Nemours, E.I., Co.) have been mainly used because they are superior in characteristics when used in fuel cells. However, these materials suffer from the problems that they are very expensive, low in heat resistance, and low in film strength, which requires some reinforcement for practical use.
Under such circumstances, development of inexpensive polymers substitutable for the above ion-conductive polymers is now hastened. Among them, those which are promising are polymers comprising an aromatic polyether excellent in heat resistance and high in film strength into which an acid group such as sulfonic acid group is introduced, namely, aromatic polymers having an aromatic main chain to which an acid group such as sulfonic acid group is directly bonded. For example, there are proposed aromatic-polyether-type ion-conductive polymers such as sulfonated polyether ketone type (JP-A-11-502,249) and sulfonated polyether sulfone type (JP-A-10-45,913 (U.S. Pat. No. 6,087,031)), JP-A-10-21,943 (U.S. Pat. No. 5,985,477)).